This invention relates to ultrasonic diagnostic imaging systems and, in particular, to ultrasonic diagnostic imaging systems which produce two dimensional (2D) and three dimensional (3D) blended multiline images.
Ultrasonic diagnostic imaging systems produce images of the interior of the body by transmitting ultrasonic waves which are steered and focused along transmit beam paths. Echoes are received from along the transmit beam path which are used to produce an image of the structure or motion encountered along the beam path. A number of adjacently transmitted beams and their echoes will interrogate a planar region of the body and the echoes can be used to produce a planar image of the body. The beams may also be transmitted adjacent to each other in three dimensions through a volumetric region, and the resulting echoes used to produce a three dimensional image of the volumetric region.
While the time required to insonify a planar region with transmit beams can be relatively short, enabling the production of 2D images of the planar region at a relatively rapid, real time rate, the time required to insonify a volumetric region can be significant. The governing factor on the time needed to insonify a volumetric region with multiple beams in three dimensions and to receive the echoes from along each of the transmit beam paths is the speed of sound, approximately 1540 m/sec. in the body. This latency is a serious limitation on the ability to perform real time, three dimensional ultrasonic imaging. An approach to addressing this limitation is to insonify the volumetric region with fewer transmit beams and to receive multiple receive beams in response to each transmit beam. This approach is known as multiline and requires a multiline beamformer which is capable of separately steering multiple receive beams in response to a single transmit beam. While multiline beamformers are in commercial use today, such beamformers typically only produce a small number of receive beams, two to six, in response to one transmit beam. Multiline beamformers for 3D imaging will need to be capable of high order multiline, where a dozen or more receive beams are produced in response to a transmit beam. The present invention is based upon the multiline technique and is especially suitable for high order multiline.
Multiline imaging is subject to several kinds of image artifacts, however. One type of artifact is the spatial artifact arising by reason of the nonuniform lateral characteristics of the insonifying transmit beams. While the beam characteristic can be made relatively flat near the beam center, the intensity rolloff at the lateral extremes of the transmit beam can affect the receive beams at those locations. While such artifacts can be removed fairly effectively for low order multiline (e.g., two receive beams for every transmit beam, or 2xc3x97multiline) by lateral spatial filtering, such filtering is unacceptable for high order multiline due to the low cut-off frequencies which result from the spatial periodicity of the greater number of receive beams.
The other type of artifact is temporal artifacts arising during imaging of moving tissue. This is always a problem during 3D cardiac imaging, and is especially noticeable at the seams of a multiline image where one group of multilines received from one transmit beam abuts another group of multilines received from an adjacent transmit beam. Accordingly it is desirable to be able to reduce these artifacts during multiline imaging, and particularly during multiline 3D imaging.
In accordance with the principles of the present invention groups of receive beams are received in response to the transmission of each of a plurality of transmit beams. Adjacent groups of receive beams spatially overlap and are detected and combined with weighting functions which vary in proportion to the spacing of a receive beam from its transmit beam. In accordance with a further aspect of the present invention, the receive beam signals are shifted in space or time prior to being combined. The inventive technique is applicable to both 2D and 3D ultrasonic imaging systems.
In the drawings:
FIGS. 1a and 1b illustrate the effect of motion on conventional single line imaging and on multiline imaging;
FIG. 2 illustrates beam profiles in conventional single line imaging;
FIG. 3 illustrates beam profiles in multiline imaging;
FIG. 4 illustrates multiline reception with no spatial overlap of received lines;
FIG. 5 illustrates multiline blending in accordance with a first embodiment of the present invention;
FIG. 6 illustrates multiline blending in accordance with a second embodiment of the present invention;
FIG. 7 illustrates multiline blending in accordance with the present invention with coherent lateral interpolation;
FIG. 8 illustrates multiline blending in three dimensional imaging in accordance with the principles of the present invention;
FIGS. 9-11 illustrates multiline blending in accordance with the present invention with weighted time shifting of blended signals; and
FIG. 12 illustrates an ultrasonic diagnostic imaging system constructed in accordance with the principles of the present invention.